The present invention relates to a metal detector for detecting metal in a harvested product flow, in particular between rotatable machine elements of harvesters, such as for example field choppers.
Manufacturers usually arrange metal detectors in a supply roller located before a cutter drum. The producers prefer supply rollers with a casing composed of a non-magnetic steel from operational as well as wear-related reasons. All metal detectors operate with permanent magnets, whose field lines extend through the roller casing and through the harvested product flow passage. One of such metal detectors is disclosed for example in U.S. Pat. No. 3,972,156. In this metal detector, a conductor is wound to form a coil, and the coil extends over the whole width of the supply roller. Such coils produce no position signal when the metal is located in the middle of the magnetic or coil arrangement.
Solutions have been proposed to eliminate this disadvantage by arranging several successive coils or coil-magnetic systems with the coils which overlap partially. The coils are connected with different magnet polarity in an addition or compensation circuit and supply a combined signal to a signal processing device. The signal processing device amplifies the produced combined signal of all coils, filters it, and compares with a threshold value which is greater than a disturbance signal caused by disturbances. If the combined signal is greater than the threshold value, the signal processing device produces a signal which activates a stopping device of the supply roller, so that the supply roller and therefore the harvested product flow is stopped in a shortest possible time.
German patent document D-OS41 40 812 discloses a device for detecting ferromagnetic foreign bodies, in which the magnetic field generation has an odd number of magnetic poles and the magnetic poles are arranged in a line transversely to the harvested product flow passage. The receiving coils are connected in series, so that the voltages induced in them are added. When a supply roller of synthetic plastic material is utilized, no disturbances caused by the supply roller occur. When the supply rollers have a casing composed of a non-magnetic steel, magnetic inhomogeneities (ferritic structures) are located in the roller casing material. They cause disturbances because of their proximity to the magnet and therefore limit the sensitivity.
A further disturbance cause is the eddy current effect. During the movement of the electrically conductive roller casing through a magnetic field, electric current is induced in it and forms a magnetic field which disturbs the detector field. Since the disturbance voltages depend on the rotary speed and the electrical resistance of the roller casing, they are the causes for a disturbance level which is difficult to detect. Thereby the sensitivity of all such circuits is limited. Also, the utilization of hall sensors and inclinedly expanding magnetic fields provide a space economy, but not any improvement of the sensitivity, as is clear for example from the European patent document EP 0666 021.
A metal detector for detecting metal foreign bodies disclosed in the European patent document EP 0702 248 provides a correct location of the metal part through the coil arrangement. Since however the coils are assembled in a star circuit and a sum signal from all coil voltages are evaluated, the sensitivity is here substantially reduced by the previously mentioned disturbance source.